This invention relates to magnetic bubble memory devices; and more particularly it relates to the fabrication of magnetic films which resist "hard bubbles" from forming in them.
One exemplary magnetic bubble memory device of the prior art, in which a magnetic film constructed according to the invention could advantageously be incorporated, is illustrated in FIG. 1. That bubble memory includes a G.sup.3 substrate 10 having a magnetic film 11 lying on one surface 10a of the substrate. Suitably, substrate 10 and film 11 respectively are 500 .mu.m thick and 2.0 .mu.m thick.
Film 11 is comprised of a crystalline iron garnet material; and the crystal has a growth induced magnetic anisotropy which produces magnetic moments throughout the film in a direction perpendicular to its surface. In operation, a bias magnetic field 12 is applied in one direction perpendicular to the film's surface. Then, each region in film 11 where a growth induced magnetic moment exists in a direction opposite to bias field 12 constitutes one magnetic bubble. Reference numeral 13 indicates one such bubble in film 11, while reference numeral 14 indicates non-bubble regions in film 11.
Also included in the prior art bubble memory device of FIG. 1 is an insulating layer 15, a current carrying conductor 16, another insulating layer 17, and bubble propagating elements 18. Suitably, layer 15 is 4500 A.degree. thick silicon dioxide; conductor 16 is 3500 A.degree. thick aluminum with 2% copper; layer 17 is 1500 A.degree. thick silicon dioxide; and propagating elements 18 are 3500 A.degree. thick nickel-iron alloys. Propagating elements 18 are patterned to define "paths" for the bubbles to follow in response to an in-plane rotating magnetic field (that is, a magnetic field which rotates in a plane parallel to the surface of film 11); and conductors 17 are patterned to define various "gates" which direct the bubbles to follow one path or another depending on the presence or absence of current in the conductor.
However, in order for magnetic bubbles in film 11 to propagate in a predictable manner along the paths defined by elements 18 and to be steered in a reliable fashion by the current in conductors 17, they must be "soft" bubbles, not "hard" bubbles. Soft bubbles are those which have no or only a few number of moment reversals around their domain wall (e.g., --two or less); whereas hard bubbles have a large number of moment reversals. One hard bubble is indicated in FIG. 1 by reference numeral 19, wherein the domain wall 20 has six moment reversals. Each location in the domain wall where a moment reversal occurs is known as a "vertical block line" 21.
One problem with hard bubbles is that they move at an unpredictable angle with respect to the rotating in-plane magnetic field. That angle varies with the number of vertical block lines in the domain wall, which is not controllable in hard bubbles. Another problem with hard bubbles is that they are less mobile than soft bubbles. Also, hard bubbles require a higher external magnetic field than do soft bubbles to cause their collapse. Thus, since hard bubbles cannot be accurately positioned, cannot be propagated at high frequencies, and are difficult to annihilate, it is highly desirable to construct film 11 such that it resists hard bubbles from forming in it.
In the past, this was achieved by constructing film 11 with materials having a negative magnetostriction coefficient; and by implanting the surface region of that film with neon atoms. These implanted atoms mechanically stressed and distorted the crystalline lattice in the surface region. Thus, due to this stress and the negative magnetostriction coefficient, a magnetic moment was induced in the surface region parallel to the film's surface. And a magnetic moment in that direction in the surface region resists hard bubbles from forming in the film.
Further, in addition to suppressing hard bubbles, it has been reported in the prior art that bubbles adhere to the boundary of an implanted magnetic moment at the film's surface. See for example, "Applications of Ion Implantation to Magnetic Bubble Devices," Journal of Vacuum Society Technology, Vol. 15, No. 5, Sept./Oct. 1978, pps. 1675-1684. Thus, by patterning the implant region, bubble "guide rails" can be formed right inside of film 11, which eliminate the need for the propagating elements 18.
But a problem with implanted magnetostrictively induced magnetic moments is that they are not thermally stable. Thus, even at relatively low processing temperatures (such as 300.degree. C., for example) the implanted atoms "gas out" of the film's surface. Then the film's crystal reverts back towards its undistorted and unstressed form; and in doing so, the magnetic moment lying parallel to the surface is almost completely dissipated.
This problem is severe because temperatures of 400.degree. C. in the implanted region may easily be exceeded by many conventional processing techniques which could be used to form the various structures that overlie the film in a bubble memory (i.e. --the oxide layers 15 and 17). Therefore, those techniques are rendered useless if the film has previously been implanted to provide a magnetic moment parallel to the film's surface.
Accordingly, the primary objects of this invention are to provide a thermally stable magnetic film, and method of fabricating the same, which resists hard bubbles from forming in it.